1. Field of Invention
The present invention relates to an improvement in a specific heat based moisture sensor, and more specifically to a specific heat based moisture sensor for measuring moisture content included in moisture bearing substances such as soil, sand, rock wool used for gardening facility, caltivation apparatus, etc.
2. Description of the Prior Art
In cultivation apparatus provided with a culture medium carrier such as rock wool, for instance, it is very important to measure and control the moisture content thereof, because the moisture content of the culture medium carrier exerts a serious influence upon the growth of cultured plants.
The same applicant has already proposed a specific heat based moisture sensor which can measure moisture content more easily and quickly than the conventional tension meter- or infrared ray-based moisture measurement instruments, in Japanese Published Unexamined (Kokai) Patent Appli. No. 63-47644.
In the specific heat based moisture sensor disclosed in the above Japanese patent document, a cylindrical metallic vessel is closed at one end and opened at the other end; wires are taken out of the open end thereof; a heater is provided within the vessel; a first high temperature sensor is arranged within the vessel a predetermined distance apart from the heater; a second low temperature sensor is arranged within the vessel without being subjected to the influence of heat of the heater; the vessel is filled with a plastic as a heat loss substance; and the wires are connected to a moisture meter.
In use of the above-mentioned specific heat based moisture sensor, the cylindrical vessel is inserted into the substance to be measured (e.g. culture medium carrier) and the heater is activated. In this case, although the heat is transmitted from the heater to the first high temperature sensor, since part of heat quantity of the substance to be measured is absorbed into the substance according to the moisture content of the substance, it is possible to measure the moisture content of the substance by measuring a change in temperature between the two temperature sensors, while keeping the heating power of the heater at a constant value.
In the above-mentioned proposed moisture sensor, since the cylindrical vessel is made of metal, there exists a serious problem in that the vessel is corroded and therefore metallic ions are produced and enter the culture medium, so that the cultured plant is subjected to a harmful influence of these ions.